1. Field of the Invention
The present invention relates generally to multiple access spread spectrum radio receivers, and more particularly to receivers with enhanced ability to acquire and track a relatively weak signal in the presence of a comparatively stronger signal.
The difference in signal strength often can be attributed to the relative distance of the signal source and the receiver, and thus the difficulty in tracking the weaker signal in the presence of a closer, stronger signal is often referred to as the near-far problem of spread spectrum multiple access. This problem can also occur when one signal source is obscured from the receiver while another signal source has a direct line of site. An example of this would be operating a receiver inside a building, perhaps near a window or a door, thereby receiving some signals at normal signal strength while others are attenuated by the building structure.
2. Description of the Prior Art
The Global Positioning System (GPS) is a radio navigation system operated by the United States Air Force for the dual purpose of providing accurate global positioning information to military as well as civilian users. To this end GPS provides two services: the Precise Positioning Service (PPS) which is available primarily to the US armed forces and requires the use of receivers equipped with the proper PPS equipment, and the Standard Positioning Service (SPS) which is less accurate than PPS but is available to all users whether or not they have access to PPS equipment. The U.S. Department of Defense has the capability to degrade the accuracy of the SPS through what is known as the Selective Availability (S/A) algorithm, and has taken an official position that all such S/A induced errors will be limited to a 100 meter horizontal position error range (2d-RMS). In contrast, PPS is accurate to within 22 meters.
GPS is essentially comprised of at least 24 satellites in orbit around the Earth at an altitude of approximately 20,000 Km in one of six orbits. Each orbit is occupied by at least four satellites. Each GPS satellite broadcasts a unique radio ranging signal that can be received by properly equipped GPS receivers. The signal contains information identifying the particular transmitting satellite and navigation data such as time and satellite position. On a fundamental level, all GPS receivers operate by tracking the ranging signals of multiple GPS satellites and determining the user's position in terms of latitude, longitude, and altitude or another equivalent spatial coordinate system.
The ranging signal broadcast by each satellite is comprised of two signals: the primary Link 1 (L1) signal broadcast at a carrier frequency of 1575.42 MHz and the secondary Link 2 (L2) signal broadcast at a carrier frequency of 1227.6 MHz. Both L1 and L2 carrier signals are spread spectrum signals modulated by digital signals, or codes, that "spread" the spectrum of each carrier signal over a specific bandwidth. The L1 signal is modulated by three bi-phase (i.e. .+-.1) digital signals: the Clear or Coarse Acquisition (C/A) Code which is a short Pseudo-Random Noise (PRN) code broadcast at a bit (or chip, which refers to each pulse of the noise code) rate of 1.023 MHz (and thus spreads the L1 carrier signal over a 1.023 MHz bandwidth by essentially breaking each bit in the original signal into 1023 separate bits, or chips, in what is known as direct sequence spread spectrum) and which therefore repeats every 1 millisecond; the Precise (P) code which is a much longer PRN code that repeats every week and is broadcast at ten times the chip rate of the C/A code (10.23 MHz); and a 50 Hz navigation data code (D). The C/A code is always broadcast in the clear (or unencrypted) whereas the P code is encrypted by an encrypting (E) code to form what is known as the Y code. The low data rate navigational code D comprises orbital parameters and clock correction information for the satellite modified by S/A.
Currently the SPS is predicated solely upon the L1 signal but in the future the SPS signal will be available on both L1 and L2. The current L1 signal contains an in-phase component modulated by P.sym.E.sym.D (where .sym. denotes the logical XOR function) and a quadrature component modulated by C/A.sym.D, and can be represented for each satellite i as ##EQU1##
where A represents the signal power, .omega. the carrier frequency, and .phi. a small phase noise and oscillator drift (i.e. clock error) component.
The broadcast satellite navigation data message D and algorithms to process it are defined in the publicly available U.S. government specification ICD-GPS-200. The satellite position portion of D is actually a prediction that is computed using ranging measurements of the GPS satellites taken at five monitoring stations distributed around the Earth. Periodically, typically daily, the GPS control segment uploads each satellite with its predicted navigation data and an estimated correction to its on-board atomic clock.
The satellite navigation data includes the GPS almanac which is used to predict the position and velocity of each GPS satellites for many weeks into the future. A typical GPS receiver uses the almanac data, the algorithms defined in ICD-GPS-200 and standard linear equation solving techniques to compute the position and velocity of each GPS satellite and to predict the expected range (PRN code phase) and Doppler frequency at which the receiver will find the satellite's signal.
Because all satellites broadcast at the same carrier frequency, each of the satellite ranging signals must be able to share this frequency with a minimum of interference from the other signals. This is accomplished by carefully selecting the PRN codes to have a sharp (1-chip wide) autocorrelation peak to enable code-synchronization and achieve equal spreading over the whole frequency band, and further have low crosscorrelation values, in a method known as Code Division Multiple Access (CDMA). The C/A PRN codes are unique to each satellite and are taken from a family of codes known as Gold codes. The GPS C/A codes are formed as the product (or modulo-2 sum) of two maximal binary code sequences (G1 and G2) each 1023 bits long. The 1023 members of this Gold code family are generated by shifting the starting state of the G2 register with respect to G1. Thirty-two out of the 1023 possible Gold codes were selected for the GPS satellites based upon two criteria: the number of ones and zeros in the code must differ by exactly one (i.e. the codes are balanced), and the crosscorrelation between any two of the C/A codes is no more than 65/1023 or -23.9 dB (normalized to the autocorrelation peak of unity). This crosscorrelation immunity is called the Gold bound, and represents the maximum interference between equal strength C/A code signals with identical frequencies. This PRN signal design enables satisfactory CDMA operation of the GPS system, i.e. as many as 32 satellites sharing the same broadcast band, provided that the received powers of the GPS signals are not larger than the Gold bound, which is typically the case.
The Gold code bound is applicable for signals with identical carrier frequencies. However, due to Doppler frequency shifts caused by motion of the satellites in their orbits and movement of the receiver, the received frequency of the GPS satellite signals is typically shifted by up to .+-.5 KHz from the nominal 1575.42 MHz L1 carrier frequency. Relative to any single satellite, the frequency of other satellites may differ by as much as .+-.9 KHz.
The strong/weak crosscorrelation problem is worse if the signals are Doppler shifted. As mentioned previously, the C/A code's Gold code family is generated by forming the mod-2 sum of a selected pair of maximal binary code sequences (G1 and G2) for all 1023 possible time shifts between the two sequences. The crosscorrelation (which is the multiplication of two signals) for binary codes is equivalent to mod-2 addition of the codes because multiplication of .+-.1 values has a one-to-one correspondence with mod-2 addition of binary 0,1 values. Therefore, the crosscorrelation of two Doppler shifted members of the Gold code family reduces to mod-2 addition of each maximal sequence with itself, followed by another mod-2 addition. The shift-and-add property of a maximal sequence means that the mod-2 sum of a maximal sequence with a shift of the same maximal sequence yields yet another shift of the same maximal sequence. Therefore, crosscorrelation of two Doppler shifted members of the Gold code family yields another member of the same Gold code family. It has been found that these generated Gold codes are not members of the C/A family and may have crosscorrelations that exceed the C/A code design limit.
No closed form analysis of the crosscorrelation interference of Doppler shifted C/A codes with relatively different carrier frequencies is known. Instead, simulations are used to analyze the effects of Doppler shifts on the crosscorrelation of C/A codes. The simulations either generate the two desired frequency offset codes and compute the crosscorrelation directly or generate the Fourier transform of each code, adjusted for frequency offset, and compute the crosscorrelation of the transforms. It has been found that for a .+-.9 KHz Doppler range the worst case crosscorrelation for the GPS C/A codes is -20.9 dB. This worst case scenario occurs when the relative Doppler shift between the two satellite signals in an integer multiple of 1 KHz.
While Doppler offsets increase the level of strong/weak signal crosscorrelation when the frequency difference is an integer multiple of 1 KHz, frequency attenuation decreases the crosscorrelation effects when the Doppler shift is not a multiple of 1 KHz. The GPS receivers integrate (sum) the in-phase and quadrature (I, Q) measurements for some length of time before they are used for signal detection or signal tracking. If the integrated signal contains a frequency error, then the accumulation decreases the signal's apparent strength by the well known sin(x)/x function, where x is half the amount of phase rotation in radians that occurs over the integration period (note that the limit of sin(x)/x is 1 as x approaches 0). Thus, if the Doppler difference between the replica weak signal and the interfering strong signal is 500 Hz and the I, Q integration time is 1 ms, then x is equal to .pi./2 radians, sin(x)/x is equal to 2/.pi. and the interference is attenuated by approximately 4 dB.
Consequently, a strong/weak signal crosscorrelation problem may occur if the strength of one satellite approaches being 20.9 dB stronger than the strength of the second satellite. Under this condition, the acquisition search may detect the crosscorrelation spectral line from the strong satellite instead of the autocorrelation spectral line from the weak satellite.
The GPS system was designed with the assumption that receivers would be operated out-of-doors with direct lines of sight to all satellites. In this case the C/A code provides adequate protection against strong/weak signal crosscorrelation. However, once a receiver moves indoors or under a canopy of trees, some of the signals can become significantly attenuated while the others continue to be received at normal signal strength. In such circumstances the operational significance of the crosscorrelation peaks of the Gold codes is to cause difficulty in being able to discriminate between a weak GPS signal and the crosscorrelation of a relatively stronger GPS signal. An incorrect discrimination may cause large errors in the latitude, longitude and altitude computed by the GPS receiver.
An SPS-equipped GPS receiver will receive at any given time the L1 ranging signals from as many as twelve satellites, all multiplexed on the same carrier frequency, each modulated by its own C/A PRN Gold code. From this compound carrier signal the receiver must be able to identify and extract the individual satellites' signals and then process each of these signals to recover the information contained therein. Each of these satellites has the potential of interfering with every other satellite signal. In a worst case, when the signals from a single weak satellite and a plurality of strong satellites are received simultaneously, the weak satellite signal may have significant crosscorrelation interference from each strong satellite signal.
When a GPS receiver is first powered on, it has at best only an approximate knowledge of its position, its local oscillator offset (which will appear as a Doppler frequency offset that is common to all satellites) and the correct time. Therefore, the receiver must perform a systematic search through a large portion of all possible C/A code phases and all possible Doppler offsets to locate the satellite signals. During the search the strong/weak crosscorrelation from any relatively strong satellite may cause the receiver to mistake a crosscorrelation spectral line from the strong satellite as a signal from a weak satellite.
After the receiver has started it can predict the C/A code phase and Doppler offset of all the satellites using the almanac data and the algorithms of ICD-GPS-200, at which time it only needs to search a relatively smaller range of C/A code phases and Doppler frequency offsets for the desired satellite signal. Nonetheless, the strong/weak crosscorrelation problem remains when the crosscorrelation peak from a relatively strong satellite occurs within the search range of a relatively weak satellite.
A typical GPS receiver consists of an antenna to receive the carrier signal while rejecting multipath and, optionally, interference signals; a preamplifier comprising a bandpass filter to filter out potential high-level interfering signals in adjacent frequency bands, and a low noise amplifier (LNA) to amplify the carrier signal; a reference oscillator to provide time and frequency reference for the receiver; a frequency synthesizer driven by the oscillator; a downconverter to convert the filtered carrier signal to an intermediate frequency (IF); an IF section to provide further filtering of out-of-band noise and interference, amplification of the signal to a workable signal-processing level, and optionally down conversion of the IF signal to a baseband signal; and an analog to digital converter (ADC) to sample and quantize the signal into in-phase (I) and quadrature (Q) components. The ADC may sample either the IF or the baseband signal, depending upon the receiver design.
The digitized I, Q signal is next fed into one to twelve or more tracking channels. There it is correlated with a C/A PRN code replica that may either be generated internally as needed with shift registers or stored as a complete set of precomputed C/A code chips in memory. The correlation of the replica and received signal is accomplished by mixing (multiplying) the two signals and integrating (summing) the power of the in-phase and quadrature components of the resultant signal. Typically, the phase of the carrier and code of the replica signal are aligned with the received signal by Phase Locked Loops (PLLs), Costas Phase Detection Loops and/or Delay Locked Loops (DLLs). The PLL and Costas loops maintain phase agreement of the received and replica signal by driving the quadrature component to zero while maximizing the in-phase component. The DLL maintains C/A code alignment by balancing the correlation power measured at two or more code offsets such as early and late or early and punctual. Each recovered spread-spectrum L1 signal is then fed to the signal processing portion of the receiver where it is demodulated to recover the signal carrier and the C/A and D codes. These in turn are supplied to a navigation data processor where the position of each satellite being tracked is computed from the D code and various error corrections are performed. Sources of error include ionospheric and tropospheric delays, the Doppler effect, satellite and receiver clock errors, equipment delays, noise, and multipath errors due to a signal being reflected and thus received multiple, but slightly delayed, times.
The maximum C/N.sub.0 (signal to noise ratio in a 1 Hz bandwidth) received by GPS receivers near the surface of the Earth is approximately 55 dB-Hz, allowing for additive multipath interference. In contrast, state of the art GPS tracking algorithms can acquire and track GPS signals with C/N.sub.9 as low as 24 dB, and future advances promise to lower this threshold even further. Thus, the range of useable GPS signal power is 35 dB or more. Assuming a worst-case strong/weak crosscorrelation C/A code spectral line of -20.9 dB, a method is needed to increase the discrimination of the C/A code by at least 10 dB-Hz.
The prior art has developed a general approach for predicting the crosscorrelation of two Doppler shifted PRN code sequences if the code timings, carrier phases, and signal amplitudes are known. The solution may be summarized as optimum maximum-likelihood demodulation of the unknown data bits by a computationally intensive Viterbi algorithm. In a practical sense, this optimum demodulation can be viewed as equivalent to doing strong signal cancellation with enough delay introduced to estimate the unknown data bits of the strong signals with a low error rate. This general solution assumes an ideal channel, but a practical solution to the near-far problem of CDMA must cope also with multipath propagation effects.
Hence, in light of the above, those skilled in the art have recognized a need to increase the strong/weak signal discrimination within CDMA coded spread spectrum signals. It has also the been recognized by those skilled in the art that it would be of value to develop such a method that will be compatible with the SPS of GPS. The present invention satisfies these needs as well as others.